Delivery fiber assembly and a broad band source

ABSTRACT

The invention comprises a delivery fiber assembly suitable for delivering broad band light and comprising a delivery fiber and a connector member. The delivery fiber has a length, an input end for launching light and a delivery end for delivering light. The delivery fiber comprises along its length a core region and a cladding region surrounding the core region wherein the cladding region comprises a cladding background material having a refractive index N bg  and a plurality of microstructures in the form of inclusions of solid material having refractive index up to N inc  and extending in the length of the longitudinal axis of the delivery fiber, wherein N inc &lt;N bg . The plurality of inclusions in the cladding region is arranged in a cross-sectional pattern comprising at least two rings of inclusions surrounding the core region. The connector member is mounted to the delivery fiber at a delivery end section of the delivery fiber comprising the delivery end. The delivery fiber has a transmission bandwidth of about 200 nm or more.

TECHNICAL FIELD

The invention relates to a delivery fiber suitable for delivering lightfrom as broad band source as well as a broad band source system, and anapparatus comprising such broad band source.

BACKGROUND ART

Broad band sources and systems are well known in the art and are forexample described in EP2081 074, WO15003714 and WO15003715.

WO15003714 discloses a supercontinuum light source comprising amicrostructured optical fiber and a pump light source, where themicrostructured optical fiber comprises an intermediate tapered section.Thereby a very broad and stable supercontinuum of light is obtained.

U.S. Pat. No. 8,731,009 discloses a super continuum light sourcecomprising a pump source and a generator fiber for generating thesupercontinuum, where the refractive index profile of the core of thegenerator fiber is arranged to allow modal cleaning of the light as itpropagates to provide an optical super continuum with relatively highspectral density and/or good beam quality.

In general most of the prior art broad band sources are focused ongenerating supercontinuum light of high quality and/or which is spanningover increasingly broader band width, e.g. supercontinuum which isspanning further into the blue wavelength e.g. below 450 nm or evenlower.

The generated light or fractions thereof are often used in highprecision illumination procedures and/or high precision measuringprocedures such as for stimulated emission depletion, for fluorescenceimaging procedures for Optical Coherence Tomography (OCT) and/or forindustrial inspection, such as metrology.

Usually the light or fractions of light generated by the broad bandsource is transmitted via a delivery fiber to an apparatus, such as anillumination apparatus and/or a measuring apparatus for use in themeasuring process of the apparatus. Generally it is desired that thedelivery fiber is simple to connect to the apparatus and it is wellknown in the art to use standard connectors such as connectors accordingto the standards IEC 61754-20, IEC 61754-15 or IEC 61754-13.

The prior art delivery fibers have usually been step index fibers havinga relatively very narrow transmission band with because such fibers areeasy to handle, easy to connectorize and have low transmission loss. Inorder to be capable of transmitting a broader band width is has beensuggested to use of photonic crystal fiber (PCF) with air holes in theircross-sections as delivery fiber has been introduced; thereby thedelivery fiber is capable of transmitting a broader band width. However,such holey PCF generally is difficult to connect and may result inundesired power loss.

DESCRIPTION OF THE INVENTION

An object of the present invention is to provide a delivery fiberassembly suitable for delivering broad band light from a broad bandsource and/or from two or more sources as well as a broad band sourcesystem where the delivery fiber is simple to connect to an apparatus fortransmitting light to the apparatus and where at least one of theproblems discussed above is alleviated.

It is further an object to provide a broad band source and a broad bandsource system comprising a delivery fiber which is simple to connecte.g. to an apparatus for transmitting light to the apparatus and whereat least one of the problems discussed above is alleviated.

In an embodiment it is an object to deliver light by the delivery fiberfrom two or more light sources having different wavelength(s).

These and other objects have been solved by the invention or embodimentsthereof as defined in the claims and as described herein below.

It has been found that the invention or embodiments thereof have anumber of additional advantages which will be clear to the skilledperson from the following description.

According to the invention it has been found that by providing a newtype of delivery fiber for delivering broad band light or selectedportions of broad band light comprising wavelengths e.g. discretewavelengths within a broad range. The delivery fiber can beconnectorized in a relatively simple and effective way and with a verylow loss. Simultaneously it has been found that the novel delivery fiberassembly has a relatively high mechanical strength, which in fact ishighly beneficial because such delivery fibers usually are subjected tomechanical disturbances such as bends and rough handling. Often thedelivery is coiled and uncoiled repeatedly and with no control ofcoiling diameter.

In an embodiment of the delivery fiber assembly of the invention it isadapted for delivering broad band light to an apparatus for transmittinglight to the apparatus. The delivery fiber assembly comprises a deliveryfiber and a connector member. The delivery fiber has a length, an inputend for launching light and a delivery end for delivering light. Thedelivery fiber comprises along its length a core region and a claddingregion surrounding the core region wherein the cladding region comprisesa cladding background material having a refractive index N_(bg) and aplurality of microstructures in the form of inclusions of solid materialhaving refractive index of up to N_(inc) and extending in the length ofand preferably along the longitudinal axis of the delivery fiber,wherein N_(inc)<N_(bg). The plurality of inclusions in the claddingregion is arranged in a cross-sectional pattern (a pattern seen in across section of the delivery fiber) comprising at least two rings ofinclusions surrounding the core region. The connector member is mountedto the delivery fiber at a delivery end section of the delivery fibercomprising the delivery end. Further, the delivery fiber has atransmission bandwidth of about 200 nm or more, such as of about 300 nmor more, such as of about 400 nm or more, such as of about 500 nm ormore for example at least an octave (halving/doubling in frequency).

Generally it is desired that the core region has a diameter up to about15 μm

In an embodiment the transmission bandwidth is defined as thewavelengths where the delivery fiber has a transmission loss of lessthan 0.5 dB/m. In an embodiment the transmission bandwidth is defined asthe wavelengths where the delivery fiber has a transmission loss of thanless than 0.1 dB/m. Advantageously the transmission loss is measuredwhen the fiber is bend with either a 16 cm or a 32 cm bending diameter.

Heretofore it has never been considered to use an all solid fibercomprising solid inclusions as delivery fiber for delivering light froma broad band source to an apparatus for measurement and/or illumination.In particular it has never been considered that such all solid fibercomprising solid inclusions could be constructed to have a transmissionbandwidth of 200 nm or larger and in particular it is surprising thatthe all solid fiber comprising solid inclusions can be constructed tohave a transmission bandwidth of 200 nm or less, below 1800 nm or evenbelow 900 nm.

The term “inclusions” means inclusions in a background material, whereinan inclusion has another refractive index than that of the backgroundmaterial surrounding it. The solid inclusions can for example beinclusions of another glass type than the background material and/or aninclusion of doped material (index changing materials such as F, Ge, P,B,) a vacuum inclusion or any combinations thereof. The inclusions of adelivery fiber can be of equal or different material or structure. Aninclusion may be of a homogeneous material or it may have regions ofdifferent materials and/or refractive index. Where an inclusioncomprises several regions of different refractive index the refractiveindex of the inclusion is determined as the average refractive index ofthe inclusion.

In the context of the present application, the phrase “ring ofinclusions” refers to the cladding inclusions typically havingsubstantially equal radial distance to the core and being aligned in aring configuration surrounding the core. Typically, a ring of inclusionsis not fully circular, but rather is shaped with a number of softangles, such as in a hexagonal shape. Preferably all the inclusions of aring of inclusions are of substantially the same size and preferably ofsame material.

The phrase “radial distance” means distance determined in radialdirection from the longitudinal axis of the core of the delivery fiber.

The term “substantially” should herein be taken to mean that ordinaryproduct variances and tolerances are comprised.

The diameter of the core and of an inclusion is determined as thecharacteristic diameter of the core/inclusion. The core or theinclusions are not always entirely circular. The characteristic diameteris the diameter of the circle of the inclusion/core where it is circularor in case the inclusion is not circular, the characteristic diameter isdetermined as the average of the maximum and the minimum extent of theinclusion/core in question.

The inclusions may have equal or different diameters and the inclusiondiameter of the respective inclusions may as mentioned be equal ordiffer along the length of the fiber.

In an embodiment the delivery end section of the delivery fibercomprising the delivery end, and the connector member is mounted at amounting distance from the delivery end such that the delivery end ispassing through the connector member for connecting the delivery end inphysical contact with a receiver waveguide, preferably a receiver fiber,such as a receiver fiber incorporated in an apparatus for illuminationand/or metrology and/or surgery.

The delivery end of the fiber is the end facet. The mounting distance tothe delivery end is usually a few mm e.g. at least 2 mm, such as up to 2cm or preferably less than 1 cm. The mounting distance to the deliveryend is usually given by the structure of the connector member.Preferably the connector member is a standard connector member such as astandard connector member according to at least one of the standards IEC61754-20, IEC 61754-15 or IEC 61754-13 or equivalent standards. Themounting distance from the delivery end is determined as the distancefrom the delivery end to the nearest physical contact to the connectormember. Usually the connector member has a connecting flange thatextends beyond the delivery end for being mated with and connected toanother connector member while simultaneously protecting the connecteddelivery end. Prior to being connected to the apparatus the delivery endis desirably protected by a removable cap. For optimal connection thedelivery end is advantageously polished and optionally arranged to havea facet which is angled e.g. up to 60 degrees, such as up to about 30degrees, such as from about 3 to 10 degrees, such as about 8 degreesrelative to the cross-section plan of the fiber so as to reduceundesired reflection of light at the end facet. For broad band operationthe angle of the facet is often selected as a compromise. Thus it isgenerally desired to have the facet angle sufficiently large enough toensure that an undesired amount of light not to be reflected and therebythat a large amount of the light is collected within the NA of thefiber. On the other hand the facet angle should advantageously not betoo large because that may result in an undesired angular dispersion.

In an embodiment the delivery end section of the delivery fiber ismounted to the connector member such that the delivery end of thedelivery fiber is suitable for connectorisation, without changing thefiber waveguide structure.

According to the present invention it has been found that the deliveryfiber of the delivery fiber assembly can be connected to an apparatuswithout the use of free space optics—i.e. without the transmission atthe connection site is outside the fiber—and without undesired change ofthe phase front of the propagating mode. Further the delivery fiber ofthe delivery fiber assembly can be connected to an apparatus withoutundesired expansion of mode field and thereby loss of light can bereduced or entirely avoided.

In an embodiment the connector member is mounted to the delivery fiberat the delivery end section of the delivery fiber at a distance to thedelivery end, such that the delivery end is passing through theconnector member for connecting the delivery end in physical contactwith a receiver waveguide, preferably a receiver fiber, such as areceiver fiber incorporated in an apparatus for illumination and/ormetrology and/or surgery.

The connector member preferably comprises a ferrule and the delivery endsection passes through the ferrule for being coupled to the receiverunit.

Advantageously the connector member is arranged such that the deliveryend can be brought in physical contact with and connected to a receivingwaveguide e.g. an optical fiber comprising a corresponding receivingwaveguide connector member.

Advantageously the connector member is spring-loaded, so the deliveryend facet can be mated to a receiving fiber in a butt coupling wherefiber faces are pressed together when the connectors are mated. Theresulting glass-to-glass contact eliminates signal losses that would becaused by an air gap between the joined fibers.

In an embodiment the delivery fiber has, at least along its delivery endsection and optionally along the entire length of the fiber, a constantmode field diameter for at least one wavelength within the transmissionbandwidth where the mode field is expanding the fiber may lose light.The delivery end section preferably comprises a length of the fiber ofat least 2 mm, preferably at least about 5 mm, such as at least 1 cm.Preferably the delivery fiber has a constant mode field diameter towavelength profile within the transmission bandwidth at least along thedelivery end section. The phrase ‘mode field diameter to wavelengthprofile’ means the profile of the respective mode field diametersrelative to the respective wavelengths within the transmissionbandwidth. Thereby the delivery fiber can be connected to an apparatusfor use with a minimum loss of light.

The mode field diameters of the delivery fiber are advantageouslyselected in dependence on the desired transmission wavelength bandwidth.It has been found that the mode field diameter for a wavelength range ofwavelengths below 2 μm varies surprisingly little. Thus for a bandwidthof 100 nm the mode field diameter varies preferably less than 10% basedon the mode field diameter of the lowest wavelength.

It has been found that the relatively small core region i.e. about 15 μmor smaller is an important factor for keeping the mode field diameterrange for the transmission bandwidth narrow. Thus, by selecting evensmaller core region the mode field diameter range for the transmissionbandwidth may be even narrower. Further it has been found that the modefield diameter variation is smaller for wavelength of about 900 nm orless than for larger wavelengths.

The delivery fiber may be tailored by selecting a mode field diameterrange i.e. In an embodiment the delivery fiber at least along saiddelivery end section has a mode field diameter range for saidtransmission wavelength, wherein the mode field diameter range is about30% or less, such as about 20% or less, such than about 15% or less,such as about 10% or less of the lowest mode field diameter of therange. The mode field diameter range is the range of mode fielddiameters corresponding to the transmission band width and the lowestmode field diameter of the range is the mode field diameter of thelowest wavelength of the transmission band width.

Advantageously the delivery fiber for at least one wavelength withinsaid transmission bandwidth and at least along said delivery end sectionhas a constant numerical aperture NA and a propagation loss less than0.5 dB/m, such as less than about 0.2 dB/m, such as less than about 0.1dB/m or even less than 0.05 dB/m.

Advantageously the delivery fiber is single mode for at least onewavelength within the transmission bandwidth. Generally the beam oflight has a much higher quality for single mode light than for multimodelight and for some applications single mode light is a requirement.Preferably the delivery fiber is single mode for at least about 50%,such as at least about 80% of the transmission bandwidth, such as forthe entire transmission bandwidth of the delivery fiber.

In a preferred embodiment the delivery fiber is apolarization-maintaining optical fiber (PM fiber). The PM fiber may forexample comprise one or more stress elements.

A PM fiber is a fiber in which linear polarization can be maintained iflinearly polarized light is launched into the fiber. Advantageously thelaunched polarized light maintains a linear polarization duringpropagation along the PM delivery fiber and exits the fiber in a linearpolarization state. Advantageously there is little or no cross-couplingof optical power between the two polarization modes. Preferably the PMdelivery fiber is single-mode for at least one wavelength within thetransmission bandwidth. By providing the delivery fiber as a PM fiber,the delivery fiber has an even lower bending loss.

The PM properties can for example be induced by incorporating stresselements e.g. such as described in U.S. Pat. No. 7,289,709.

The core region may in principle have any core diameter depending on thepower and the wavelengths to be transmitted. Advantageously the deliveryfiber core region has a diameter of at least about 3 μm. The core regionmay vary along the length of the delivery fiber, but generally it ispreferred that the core region has a substantially identical diameteralong the major part of the delivery fiber, such as preferably along atleast about 80% of the length of the delivery fiber. The most suitablecore region diameters are up to about 15 μm, such as within the range offrom about 3 μm to about 10 μm, such as from about 4 μm to about 12 μm.

Where the transmission bandwidth of the delivery fiber compriseswavelengths below 500 nm or even below 450 nm, the delivery fiber coreregion advantageously has a diameter of less than about 10 μm because ithas been found that where the core region of the delivery fiber islarger, such as larger than about 15 nm, there is an increased loss inthe UV light region.

All structural details of the optical fiber, such as core region size,inclusion diameters and pattern are given in relation to across-sectional view of the fiber unless otherwise specified.

In an embodiment the core region diameter is substantially identicalalong the length of the delivery fiber, optionally excluding one or morecomponent sections of the delivery fiber.

Such component section(s) is/are further described below.

Preferably the plurality of inclusions in the cladding region isarranged in a cross-sectional pattern comprising at least four rings ofinclusions surrounding the core region, such as at least five rings ofinclusions surrounding the core region. It has been found that byincreasing the number of rings of inclusion the transmission bandwidthcan be increased and further additional properties, such as single modeproperties and/or reduced loss can be enhanced.

The rings of inclusions are advantageously arranged in a pitch patternsuch that the distance between adjacent inclusions is P times the pitch,where P is 0.5 or an integer up to five, preferably up to 3, such as 1or 2, and where P can have different value from different distances ofnearest inclusions.

In an embodiment the inclusions are arranged in a pitch pattern, adouble pitch pattern, a triple pitch pattern, a half pitch pattern or acombination thereof, where a pitch is determined as the smallest centerto center distance between the core region and an inclusion.

It has been found that the pitch ∧ can be used in tailoring the deliveryfiber for a desired transmission band width and in particularly whetheror not the transmission band width comprises wavelength in the bluerange below 450 nm and/or in the red range above 800 nm. To reach intothe blue wavelengths it is desired that the pitch is less than about 10μm, preferably less than about 9 μm, such as less than about 8 μm, suchas less than about 7 μm, such as less than about 6 μm. To reach into thered wavelength the pitch is advantageously about 3.2 μm or larger, suchas about 3.2 μm or larger, such as about 4 μm or larger, such as about 5μm or larger, such as about 5 μm or larger, such as about 7 μm orlarger, such as about 8 μm or larger.

The core region diameter to pitch d/∧ may advantageously be about 0.7 orless, such as from about 0.3 to about 0.65, such as from about 0.4 toabout 0.6.

Advantageously the core region has a refractive index N_(core), thecladding region has an effective refractive index N_(clad), and therefractive index N_(bg) of the cladding background material issubstantially identical to the refractive index of the core regionN_(core).

In an embodiment the refractive index of the core region is homogeneous.In another embodiment the core region is microstructured e.g., asdescribed in co-pending DK PA 2014 00545. Where the core region ismicrostructured the refractive index of the core region Ncore isdetermined as the average refractive index.

In an alternative embodiment the refractive index N_(bg) of the claddingbackground material differs from the refractive index of the core regionN_(core), preferably such that the refractive index N_(bg) of thecladding background material is lower than the refractive index of thecore region N_(core).

In an embodiment the delivery fiber is a silica fiber and the solidinclusions are of down doped silica. Preferably the solid inclusionsindependently of each other are silica doped with at least one offluorine and/or boron and/or component comprising F and/or B atoms.

The respective inclusions may have equal or different refractive indicesand an inclusion may have several regions with different refractiveindices in which situation the refractive index is determined as theaverage refractive index as described above.

In an embodiment, the difference between refractive indices of therespective inclusions independent from each other and the refractiveindex of the background material N_(bg) is from about 10⁻⁵ to about 0.1,such as from about 10⁻⁴ to about 10⁻², such as up to about 10⁻³.

In an embodiment the inclusions have substantially identical refractiveindex N_(inc).

In an embodiment n of the inclusions have different refractive indicesN_(inc(1)) . . . , N_(inc(n)), where n is an integer up to the number ofinclusions—in other words in principle all of the inclusions may havedifferent refractive indices. However, from a practical point of viewthis embodiment is not preferred.

Advantageously up to 10, such as 2-4, preferably all of the inclusionsof a ring of inclusion have substantially identical refractive indices.

In an embodiment the inclusions of one ring of inclusions have arefractive index different from that of the inclusions of another ringof inclusions. Preferably the inclusions of the one ring of inclusionshave higher refractive index than the inclusions of another ring ofinclusions closer to the core region. By having rings of inclusions withdifferent refractive indices, the delivery fiber can be designed to havea desired transmission bandwidth and at the same time higher order modesmay be suppressed.

In an embodiment the inclusions of a ring of inclusions comprises two ormore inclusions having different refractive index.

Advantageously the solid inclusions are substantially parallel to thecore region. Thereby the fiber can be drawn in a relatively simple way.

In an embodiment the solid inclusions are helically surrounding the coreregion. Thereby the number of inclusions can be reduced and/or thebending loss can be reduced. However, the production of fiber withinclusions helically surrounding the core region is more difficult thanwith inclusions parallel to the core region.

In an embodiment the delivery fiber comprises an inner cladding regionand an outer cladding region, where the cladding background material maydiffer.

In an embodiment the delivery fiber comprises an inner cladding regionand an outer cladding region, where the cladding background material hasidentical refractive index.

In an embodiment the delivery fiber comprises an inner cladding regionand an outer cladding region, where the inclusions in the outer claddingregion have higher index than the inclusions in the inner claddingregion. In this embodiment the inner cladding region advantageouslycomprises the herein described inclusions of solid material havingrefractive index of up to N_(inc) and the inclusions in the outercladding region are of solid material having refractive index which ishigher than N_(inc). Preferably the inclusions in the outer claddingregion are of solid material having refractive index which is higherthan the background material of the outer cladding region. Thereby theinclusions of the outer cladding region may act to couple out higherorder modes.

In an embodiment the delivery fiber is a double clad delivery fibercomprising an inner cladding region and an outer cladding region forexample as described above, and wherein the delivery fiber is configuredsuch that signals can be collected e.g. via free space optic or via acoupler e.g. a fused coupler as described below.

The inclusion may in principle have any size, but generally it isdesired that the inclusions are not too large unless where inclusionsare used for making the fiber birefringent.

In an embodiment the solid inclusions have equal or different diameters,preferably the diameters is from about 0.2 to about 1 μm, such as fromabout 0.4-0.8 μm.

Preferably the inclusions of a ring of inclusions have equal diameter.

In an embodiment the inclusions in a ring of inclusions have a firstdiameter and the inclusions in another ring of inclusions have a seconddiameter that differs from the first diameter.

Advantageously the delivery fiber is an all solid fiber—i.e. a fiberentirely of solid material at 25° C. Preferably the delivery fiber is anall silica fiber, wherein the core region, the cladding region and/orthe inclusions are doped to reach their respective refractive indices.

In an embodiment the transmission bandwidth of the delivery fibercomprises wavelengths within the range of from about 400 nm to 900 nm.This range of wavelength is highly suitable for a plurality of highprecision procedures, in particularly for microscopy based procedures.Preferably the transmission bandwidth of the delivery fiber comprises atleast a bandwidth of about 100 nm, such as at least a bandwidth of about200 nm such as at least a bandwidth of about 300 nm, such as at least abandwidth of about 400 nm, such as the entire bandwidth within the rangeof from about 400 nm to 900 nm.

In an embodiment the transmission bandwidth comprises at least onewavelength in the visible range.

In an embodiment the transmission bandwidth comprises wavelengths abovethe visible range, for example of 700 nm or larger. This is highlysuitable for illumination during eye surgery, in particular wheretransmission bandwidth is exclusively above 700 nm.

In an embodiment the transmission bandwidth comprises at least onewavelength below 450 nm, or even below 400 nm. Such wavelengths are verysuitable for many microscopy illumination procedures.

In an embodiment the delivery fiber is configured for suppressingwavelengths above 1500 nm. This can be provided by the selection of thedistance between the inclusions in the respective rings of inclusionsand/or by the selection of the refractive index difference between therefractive index of the core region and the effective refractive indexof the cladding region.

In an embodiment the delivery fiber is configured for suppressingwavelengths above 900 nm.

In an embodiment the transmission bandwidth of the delivery fibercomprises wavelengths within the range of from about 1100 nm to 2400 nm,preferably the transmission bandwidth of the delivery fiber comprises atleast a bandwidth of about 100 nm, such as at least a bandwidth of about200 nm such as at least a bandwidth of about 300 nm, such as at least abandwidth of about 400 nm, such as the entire bandwidth within the rangeof from about 1100 nm to 2400 nm.

In an embodiment the transmission bandwidth of the delivery fibercomprises wavelengths within the range of from about 400 nm to 1100 nm.

In an embodiment the transmission bandwidth of the delivery fibercomprises wavelengths below 800 nm, such as below 700 nm, such as below600 nm, such as below 500 nm.

The delivery fiber should not be too long because this may result inundesired bends and mechanical disturbance. However, neither should thedelivery fiber be too short. In preferred embodiments the delivery fiberassembly can be supplied in different lengths for the specific use.

A suitable length of the delivery fiber is for example from about 5 cmto about 100 m, such as from about 10 cm to about 30 m, such as fromabout 20 cm to about 20 m, such as from about 30 cm to about 10 m.

Advantageously the delivery fiber comprises at least one componentsection, the component section is advantageously configured forsplitting of light from the delivery fiber and/or for combining light inthe delivery fiber, the component section preferably is an all fibercomponent section.

Advantageously the delivery fiber comprises a fused component, such as afused coupler or a fused splitter. Such fused components are well knownin the art, but have heretofore never been applied or fused to adelivery fiber.

The principle of a fused element is that the core of the delivery fiberand the core of the fused component—usually a fiber component—are fusedto be very close to each other so as to transfer light from one core toanother. An advantage of the fused component(s) comprises an opticalfiber with solid microstructures e.g. comprising a core region and acladding region surrounding the core region wherein the cladding regioncomprises a cladding background material having a refractive indexTN_(bg) and a plurality of microstructures in the form of inclusions ofsolid material having refractive indices of up to TN_(inc) and extendingin the length of and preferably along the twin delivery fiber, whereinTN_(inc)<TN_(bg) and the plurality of inclusions in the cladding regionis arranged in a cross-sectional pattern comprising at least two ringsof inclusions surrounding the core region.

It has been found that by using all solid fibers comprising solidmicrostructures in form of inclusions as described, the coupling orsplitting of light to or from the delivery fiber will be such that thefraction of light coupled to or split from the delivery fiber has asubstantially identical wavelength profile as the light from where itwas coupled or split.

In an embodiment the delivery fiber assembly comprises a fused coupler,wherein the fused coupler is fused to the delivery fiber at thecomponent section.

Advantageously the fused coupler is a 2×2 coupler, comprising a couplingfiber fused to the delivery fiber at the component section. The couplingfiber comprises a first and a second fiber section on either side of thefused component section and is arranged such the light is coupledbetween the delivery fiber and the coupling fiber at the fused componentsection.

In an embodiment the coupling fiber is arranged for coupling light intothe delivery fiber and simultaneously coupling a light fraction from thedelivery fiber to the coupling fiber. Such fused coupler is advantageousfor use in OCT as shown in the examples below.

In an embodiment the delivery fiber assembly comprises a fused splitter,wherein the fused splitter is fused to the delivery fiber at thecomponent section for splitting a fraction of light from the deliveryfiber. Preferably the split off light fraction constitutes from 5-95% bypower of the light, wherein the split off light fraction has awavelength profile substantially identical to the light fraction fromwhich it was split.

In an embodiment the splitter is a 10-90% ratio splitter in that thesplit off light will constitute about 10% by power or about 90% bypower. This type of splitter is e.g. suitable for use in OCT or wafermetrology or similar apparatus.

In general the splitter can be applied for splitting any desired powerfraction of light from the delivery fiber and the split off fraction canbe used as a reference. In an embodiment the delivery fiber with thesplitter is configured for use in an interferometer apparatus.

In an embodiment the splitter is a 50-50% ratio splitter in that thesplit off light will constitute about 50% by power. This embodiment maybe applied for providing interleaved pulses, e.g. as described inAdvanced Fluorescence Fluctuation Spectroscopy with Pulsed InterleavedExcitation, Matthias Höller, Dissertation zur Erlangung des Doktorgradesder Fakultät für Chemie and Pharmazie der Ludwig-Maximilians-UniversitatMünchen, 2011.

In an embodiment the fused splitter comprises a twin delivery fiber fordelivering light, the twin delivery fiber comprises a twin delivery endand a length extending from the component section to the twin deliveryend and comprises along its length a core region and a cladding regionsurrounding the core region wherein the cladding region comprises acladding background material having a refractive index TN_(bg) and aplurality of microstructures in the form of inclusions of solid materialhaving a refractive index of up to TN_(inc) and extending in the lengthof and preferably along the twin delivery fiber, whereinTN_(inc)<TN_(bg) and the plurality of inclusions in the cladding regionis arranged in a cross-sectional pattern comprising at least two ringsof inclusions surrounding the core region, preferably a twin connectormember being mounted to the twin delivery fiber at a twin delivery endsection of the twin delivery fiber comprising the twin delivery end.

Advantageously the delivery fiber has a transmission bandwidth of about200 nm or more, such as of about 300 nm or more, such as of about 400 nmor more, such as of about 500 nm or more.

In an embodiment the delivery fiber assembly comprises a fused couplerand a fused splitter, wherein the fused splitter is fused to thedelivery fiber at a first component section and the fused coupler isfused to the delivery fiber at a second component section closer to thedelivery end of the delivery fiber, the fused splitter and fused couplercomprises a loop fiber configured for delaying a light pulse fractionsplit out via the splitter and recombined into the delivery fiber viathe coupler such that a split off and recombined light pulse fraction isdelayed relative to the light pulse fraction from which it was split.

The loop fiber is advantageously an all solid fiber comprisinginclusions e.g. as described above.

Advantageously the assembly comprises an input end connector member andthe input end connector member is mounted to the delivery fiber at aninput end section of the delivery fiber comprising the input end, theconnector member preferably being configured for connecting the inputend in physical contact with a light launching unit, such as amicrostructured optical fiber.

In an embodiment the assembly comprises a pre-delivery fiber, thepre-delivery fiber is coupled to the delivery fiber without free spaceoptics.

In an embodiment the pre-delivery fiber is spliced to the deliveryfiber, optionally by a splicing.

Advantageously the splicing is provided such that guiding one or moreproperties selected from mode field diameter, numerical aperture, lowpropagation loss or transmission band width are substantially unalteredfrom the pre-delivery fiber to the delivery fiber.

In an embodiment the delivery fiber assembly is configured fordelivering light with single mode quality from a multiplum ofmultiplexed lasers with different wavelengths. Such multiplexed laserlight is sometimes referred to as a spectral engine. The multiplexedlaser light may e.g. be multiplexed from a blue, a green and a red laserwhich are combined into a single beam path with e.g. a KeyopticsKineFLEX module. Prior art would either be using three differentdelivery fibers or having a single delivery fiber which was single modeat the large wavelengths and multimode at the low wavelengths.Heretofore there has not been any solution that makes it possibly todeliver single mode multiplexed light comprising different wavelengths,such as wavelengths that differs with about 25 nm or more, such as about100 nm or more, such as 200 nm or more, such as 300 nm or more by onesingle delivery fiber.

The invention also relates to a broad band source for supplying light toan apparatus. The broad band source comprises

-   -   an optical pump source operable to generate pump pulses    -   a microstructured optical fiber for generating broad band light        pulses upon feeding of pump light; and    -   a delivery fiber arranged for receiving at least a portion of at        least some of the broad band light pulses and for delivering at        least a part of the received portion of the broad band light        pulses to the apparatus, wherein the optical pump source is        arranged to launch pump pulses to the microstructured optical        fiber, and the delivery fiber has a length, an input end for        launching light and a delivery end for delivering light, the        delivery fiber comprises along its length a core region and a        cladding region surrounding the core region wherein the core        region has a diameter of up to about 15 μm and the cladding        region comprises a cladding background material having a        refractive index N_(bg) and a plurality of microstructures in        the form of inclusions of solid material having a refractive        index of at least N_(inc) and extending in the length of and        preferably along the longitudinal axis of the delivery fiber,        wherein N_(inc)<N_(bg) and the plurality of inclusions in the        cladding region is arranged in a cross-sectional pattern        comprising at least two rings of inclusions surrounding the core        region.

The delivery fiber of the broad band source may advantageously be as thedelivery fiber described above.

In an embodiment the delivery fiber has a transmission bandwidth ofabout 200 nm or more, such as of about 300 nm or more, such as of about400 nm or more, such as of about 500 nm or more.

Advantageously, the delivery fiber has a constant mode field diameterfor at least one wavelength within the transmission bandwidth at leastalong the delivery end section, preferably the delivery fiber has aconstant mode field diameter to wavelength profile within thetransmission bandwidth at least along the delivery end section. In oneembodiment the delivery fiber has a substantially constant mode fielddiameter along its entire length including the delivery end section withthe connector.

Advantageously the delivery fiber has a substantially constant modefield diameter to wavelength profile along the major part of its length,preferably as described above e.g. at least including the end sectionsincluding 1 cm from delivery end.

The delivery fiber is preferably single mode for at least one wavelengthwithin the transmission bandwidth. Preferably the delivery fiber issingle mode for at least about 50%, such as at least about 80% of thetransmission bandwidth, such as for the entire transmission bandwidth ofthe delivery. The delivery fiber preferably has a transmission bandwidthof about 200 nm or more, such as of about 300 nm or more, such as ofabout 400 nm or more, such as of about 500 nm or more.

Advantageously the delivery is a polarization-maintaining optical fiber(PM fiber) as described above.

In an embodiment the plurality of inclusions in the cladding region isarranged in a cross-sectional pattern comprising at least four rings ofinclusions surrounding the core region, such as at least five rings ofinclusions surrounding the core region. The rings of inclusions arearranged in a pitch pattern, a double pitch pattern, a triple pitchpattern, a half pitch pattern or a combination thereof, e.g. asdescribed above.

In an embodiment the fiber has a so called single cell core, whichbasically corresponds to a single missing inclusion. In this embodimentthe core size is given by two times the pitch (∧) minus the diameter ofthe inclusions (d).

Preferably the core region has a refractive index N_(core), the claddingregion has an effective refractive index N_(clad), and the refractiveindex N_(bg) of the cladding background material is substantiallyidentical to the refractive index of the core region N_(core).

The delivery fiber is advantageously a silica fiber and the solidinclusions are of down doped silica, preferably the solid inclusionsindependently of each other are silica doped with at least one offluorine and/or boron and/or component comprising F and/or B atoms.

In an embodiment the delivery fiber is a double clad fiber comprising aninner cladding region and an outer cladding region, where the inclusionsin the outer cladding region have higher indices than the inclusions inthe inner cladding region, preferably the inner cladding regioncomprises the at least two rings of inclusions of solid material havinga refractive index of at least N_(inc) and the outer cladding region hasa higher effective refractive index than the inner cladding region.

The inclusions in the outer cladding region are preferably of solidmaterial having a refractive index which is higher than N_(inc),preferably the inclusions in the outer cladding region are of solidmaterial having a refractive index which is higher than the backgroundmaterial of the outer cladding region. Thereby the inclusions of theouter cladding may act to couple out higher order modes.

In an embodiment the outer cladding is arranged to receive and transmita signal from an illuminated target. The light scattered from the targetmay be collimated by an optical element to be transmitted via the outercladding e.g. to a spectrometer.

The delivery fiber is advantageously in form of a delivery fiberassembly as described above.

The optical pump source can in principle be any kind of optical pumpsource, e.g. such as the optical pump source described in WO2011/023201. Advantageously the optical pump source comprises amode-locked fiber oscillator and preferably at least one amplifier. Inan embodiment the optical pump source comprises a master oscillatorpower amplifier MOPA. MOPA configurations are well known in the art.

The microstructured optical fiber can be any microstructured opticalfiber suitable for supercontinuum generation e.g. as described in theabove cited prior art documents.

Advantageously the optical pump source and the microstructured opticalfiber of the broad band source are configured for generating broad bandlight pulses spanning at least about 200 nm, preferably at least about500 nm, such as an octave.

Advantageously the microstructured optical fiber comprises holes whichare connected to a connector member connected to the delivery fiberpreferably without the use of free space optics.

In an embodiment the microstructured optical fiber comprising holes isspliced to the delivery fiber e.g. as described in US2012195554.

In an embodiment the microstructured optical fiber is connected to thedelivery fiber by butt connection, optionally by connector members or bysplicing, such as a cold splice or where the holes are collapsed at theend of the microstructured optical fiber.

In an embodiment the microstructured optical fiber is connected to thedelivery fiber by using of a gradient index fiber arrangement (GRIN) asdescribed in “Low-Loss High-Strength Microstructured Fiber FusionSplices Using GRIN Fiber Lenses” by A. D. Yablon and R. Bise. 2004Optical Society of America, OCIS codes: (060.2310) Fiber optics.

In an embodiment the broad band source comprises a band pas filter forfiltering the broad band light pulses, the band pas filter is preferablya tunable band pass filter, preferably selected from a grating basedfilter, a prism, or an acousto-optic tunable filter (AOTF). Such bandpass filters are well known to the skilled person.

Advantageously the band pass filter is positioned between themicrostructured optical fiber and the delivery fiber.

In an embodiment the delivery fiber is arranged to receive the band passfiltered portion of the broad band light pulses and for delivering theat least a part of the received portion of the broad band light pulsesto the apparatus without free space optics.

In an embodiment the filter is a tunable optical band pass filter,configured for selectively selecting a plurality of wavelengths orwavelength ranges and transmitting the selected wavelengths to thedelivery fiber.

In an embodiment wherein the pump source is operable to generate pumppulses at a pump pulse repetition rate, and the broad band sourcecomprises a pulse picker arranged between the pump source and themicrostructured optical fiber, the pulse picker being operable to pickout pulses to reduce the pump pulse repetition rate generated by thepump source, such that the reduced pump pulse repetition rate islaunched to the microstructured optical fiber.

The invention also comprises a broad band source system comprising thebroad band source as described above and wherein the broad band sourcesystem comprises a plurality of delivery fiber assemblies havingdifferent transmitting properties, wherein each of the delivery fiberassemblies comprises an input end connector member and a delivery endconnector member. Thereby the user can select a delivery fiber assemblyfor a specific use and switch to another delivery fiber assembly foranother use. The respective delivery fiber assemblies are advantageouslydesigned for the performance of different procedures and the user canrelatively simply switch delivery fiber assembly.

The invention also relates to a spectral engine source for supplyinglight to an apparatus, the spectral engine source comprising

-   -   two or more lasers emitting laser beams with wavelength(s) that        differs with at least one wavelength    -   a multiplexer, and    -   a delivery fiber

Thus the spectral engine may comprise a multiplum of lasers emittinglaser beams with distinct wavelength or band of wavelength.

The multiplexer is configured for receiving at least a portion of thelaser beams of each of the lasers and for collimating the received lightto a multiplexed beam. The delivery fiber is arranged to receive themultiplexed beam and to delivering at least a part of the receivedmultiplexed beam to the apparatus, and wherein the delivery fiber has alength, an input end for launching light and a delivery end fordelivering light. The delivery fiber comprises along its length a coreregion and a cladding region surrounding the core region wherein thecore region has a diameter of up to about 15 μm and the cladding regioncomprises a cladding background material having a refractive indexN_(bg) and a plurality of microstructures in the form of inclusions ofsolid material having refractive index of up to N_(inc) and extending inthe length of the longitudinal axis of the delivery fiber, whereinN_(inc)<N_(bg) and the plurality of inclusions in the cladding region isarranged in a cross-sectional pattern comprising at least two rings ofinclusions surrounding the core region, preferably the delivery fiberhas a transmission bandwidth comprising wavelengths comprising at leasta part of the wavelengths of each of the two or more lasers.

The delivery fiber may be as described above.

The lasers may be any kind of lasers, such as fiber lasers orsemiconductor diode lasers.

In an embodiment at least one of the laser beams emitted from the two ormore lasers have a bandwidth of about 50 nm or less, such as about 25 nmor less, such as about 5 nm or less.

In an embodiment one of the laser beams emitted from the two or morelasers comprises at least one wavelength below 500 nm and one of thelaser beams emitted from the two or more lasers comprises at least onewavelength above 800 nm.

In an embodiment the spectral engine comprises at least three lasers,wherein a first laser of the lasers is adapted for emitting a laser beamcomprising at least one wavelength below 450 nm, a second laser of thelasers is adapted for emitting a laser beam comprising at least onewavelength in the range from 500 nm to 700 nm, and a third of the lasersis adapted for emitting a laser beam comprising at least one wavelengthabove 800 nm.

The invention also relates to an apparatus comprising a broad bandsource or a spectral engine source comprising as described above. Theapparatus comprises an optical waveguide arranged for receiving lightfrom the delivery fiber, wherein the optical waveguide comprises aconnector member configured for being mated with the delivery endconnector member of the delivery fiber, the optical wave guidepreferably being an optical fiber.

The apparatus is for example an illumination apparatus configured forilluminating a target, the illumination apparatus is preferably selectedfrom a microscope, a spectroscope or an endoscope.

Advantageously the illumination source is adapted for fluorescenceImaging; Fluorescence Lifetime Imaging (FLIM); Total Internal ReflectionFluorescence (TIRF) Microscopy; fluorescence resonance energy transfer(FRET); pulse interleave excitation foster resonance energy transfer(PIE-FRET); broadband Spectroscopy; nanophotonics; flow cytometry;industrial inspection, such as metrology; ringdown spectroscopy, such asgas sensing; analytical spectroscopy, such as hyperspectralspectroscopy, crop analysis e.g. of fruits and time of flightspectroscopy (TCSPC); single Molecule Imaging and/or combinationsthereof. In one embodiment the delivery fiber serves as a so-calledlight guide in the microscope.

In an embodiment the apparatus is a metrology apparatus, the apparatuspreferably comprises a double clad delivery fiber configured forcollecting signals from an illuminated target.

In an embodiment the apparatus comprises at least one delivery fiberassembly comprising a fused coupler and/or at least one delivery fiberassembly comprising a fused splitter.

In an embodiment the illumination source comprises at least one deliveryfiber assembly comprising a fused coupler and/or at least one deliveryfiber assembly comprising a fused splitter and wherein the deliveryfiber has a transmission bandwidth of at least about 200 nm, preferablyat least about 500 nm, such as an octave.

All features of the inventions and embodiments of the invention asdescribed above including ranges and preferred ranges can be combined invarious ways within the scope of the invention, unless there arespecific reasons not to combine such features.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and/or additional objects, features and advantages of thepresent invention, will be further elucidated by the followingillustrative and non-limiting detailed description of embodiments of thepresent invention, with reference to the appended drawings.

FIG. 1a shows a cross-section of a delivery fiber of an embodiment of adelivery fiber assembly of the invention.

FIG. 1b shows a wavelength profile in form of a transmission lossspectrum for a delivery fiber as shown in FIG. 1 a.

FIG. 1c shows an example of a PM fiber, where the dark regions areinclusions doped with another material, such as e.g. boron

FIG. 1d shows an example of the long wavelength transmission edge andshort wavelength transmission edge versus the pitch for a fiberaccording to the invention.

FIG. 2 shows a perspective view of a part of an embodiment of thedelivery fiber assembly of the invention.

FIG. 3 is a schematic illustration of an embodiment of a broad bandsource of the invention.

FIG. 4 is a schematic illustration of another embodiment of a broad bandsource of the invention.

FIG. 5 is a schematic illustration of yet another embodiment of a broadband source of the invention.

FIG. 6 is a schematic illustration of an embodiment of an apparatus ofthe invention in form of an interferometer.

FIG. 7 is a schematic illustration of an embodiment of an apparatus ofthe invention in form of another type of interferometer.

FIG. 8 is a schematic illustration of an embodiment of a spectral enginesource of the invention.

FIG. 9 is a schematic illustration of another embodiment of a spectralengine source of the invention.

FIG. 10 is an illustration of an apparatus of an embodiment of theinvention

FIG. 11 is an illustration of another apparatus of an embodiment of theinvention

The figures are schematic and may be simplified for clarity. Throughout,the same reference numerals are used for identical or correspondingparts.

FIG. 1a shows a cross-section of a delivery fiber 5 of a delivery fiberassembly. The delivery fiber 5 comprises a core region with a refractiveindex n_(core). The core is surrounded by a cladding region comprising abackground material having a refractive index N_(bg) and a plurality ofmicrostructures in the form of inclusions 2 a, 2 b of solid materialhaving refractive index up to N_(inc) and extending in the length of thelongitudinal axis of the delivery fiber, wherein N_(inc)<N_(bg). Theplurality of inclusions 2 a, 2 b in the cladding region is arranged in across-sectional pattern comprising at least 6 rings of inclusionssurrounding the core region. In the shown embodiment, the 3 in radialdirection innermost rings of inclusions 2 a have a lower refractiveindex than the 3 in radial direction outermost rings of inclusions 2 b.In the shown embodiments the inclusions have substantially equaldiameter. As explained above it may for some applications beadvantageous to have different diameters for one ring of inclusionsrelative to another ring of inclusions.

FIG. 1b shows a transmission loss spectrum of a fiber corresponding tothe delivery fiber shown in FIG. 1 with six rings of inclusions, butwhere all inclusions have same refractive index. In this example thecore has a diameter of 10 μm, the inclusions have a refractive indexwhich is 1.2% lower than the refractive index of the core and thecladding background silica material at 635 nm. The center to centerdistance between the inclusions in the cladding (also known as thepitch) is 6 μm and the diameter of the inclusions is 3 μm.

The spectrum is obtained by sending light from a broad band spectrumlight source through the fiber and performing a cut-back to separate thecoupling loss. The peak at around 1400 nm is due to water absorption.The delivery fiber is single-mode in the entire transmission bandwidthof the delivery fiber. It can be seen that the delivery fiber has a verybroad transmission bandwidth extending from about 425 nm to about 1500nm.

The delivery fiber shown in FIG. 1c is an example of a PM fibercomprising a core region with a refractive index n_(core). The core issurrounded by a cladding region comprising a background material havinga refractive index N_(bg) and a plurality of microstructures in the formof inclusions 7 a, 8 b of solid material having refractive index up toN_(inc) and extending in the length of the longitudinal axis of thedelivery fiber, wherein N_(inc)<N_(bg). The plurality of inclusions 7 a,7 b in the cladding region is arranged in a cross-sectional patterncomprising at least 6 rings of inclusions surrounding the core region.In the shown embodiment, a number of inclusions b with a higherrefractive index than the other inclusions 7 a is arranged in twoopposite clusters for forming stress elements. The inclusions 7 b withthe higher refractive index are e.g. doped with boron.

In an embodiment of the of the delivery fiber is has been found that thediameter (d) of the inclusions in the cladding relative to the pitch (∧)may have large influence on the guiding properties of the deliveryfiber. In an embodiment the transmission loss is large if the d/∧ isless than about 0.4. In an embodiment the fiber is multimode if d/∧ ismore than about 0.6.

In an embodiment of the invention is has been found that the pitch (∧)of the delivery fiber may have large influence on the spectraltransmission bandwidth of the delivery fiber.

In particular it was found that the short wavelength transmission edgesets a upper limit to the pitch and where this limit increases with therequired short wavelength edge, such that e.g. a short wavelength edgeof 300 nm requires a pitch of less than 6 μm whereas a short wavelengthedge of 600 nm requires a pitch of less than 9 μm.

Further it was found that the long wavelength transmission edge sets alower limit to the pitch and where this limit increases with therequired long wavelength edge, such that e.g. a long wavelength edge of800 nm requires a pitch of at least 3.2 μm whereas a long wavelengthedge of 1500 nm requires a pitch of at least 6 μm.

FIG. 1d shows the pitch shows an example of the long wavelengthtransmission edge (201) and short wavelength transmission edge (202)versus the pitch for a fiber according to the invention having a d/∧ ofabout 0.5.

In an example the mode field diameter of the delivery fiber varies fromabout 8.0 μm at 500 nm to about 9.0 μm at 900 nm. In an embodiment themode filed diameter of the fiber varies by less than about 20% from 500nm to 900 nm.

FIG. 2 shows a part of a delivery fiber assembly comprising the deliveryfiber 5 and a connector member 6 mounted to the delivery fiber at adelivery end section of the delivery fiber comprising the delivery end 5a. As it can be seen the delivery fiber comprises a protection coating,such as a polymer protection coating.

The connector member is advantageously an optical fiber connector of thetype which in the prior art is commonly used to terminate the end of anoptical standard fiber to enable easy connection and disconnection oftwo standard optical fibers with low loss.

During mounting, the optical fiber is typically aligned inside theoptical connector member, so that the core region of the opticaldelivery fiber is centered inside a connector plane of the connectormember. For a polarizing or polarization maintaining fiber it is alsopossible to rotate the fiber so that the polarization axis is in apredetermined plane. Furthermore it is ensured that the end facet of thefiber is in the output plane of the connector member. This can e.g. beachieved by polishing the connector and fiber end facets.

If light is sent through the delivery fiber assembly with a connectormember on the output end, then the position of the light being emittedfrom the output of the connector member is well known. For a standardall-solid single mode fiber the light will have its focal plane andthereby waist at the output plane of the connector.

Many different types of connectors have been introduced to the marked,such as e.g. FC, E-2000, SMA connectors, as well as connectors withbuilt in beam expansion.

It is desired to use fiber connector members having low loss and highpower handling, as they should advantageously be capable of handlingaverage powers such as up to 100 mW or even up to several Watts.

The broad band source 10 shown in FIG. 3 comprises a broad band laserpulse generator 1 comprising a not shown optical pump source operable togenerate pump pulses and a not shown microstructured optical fiber forgenerating broad band light pulses upon feeding of pump light where theoptical pump source is arranged to launch pump pulses to themicrostructured optical fiber. In the shown embodiment the broad bandlaser pulse generator 1 is in the form of a SuperK™ system marketed byNKT Photonics Denmark. The broad band source further comprises adelivery fiber 5 comprising solid inclusions as described above. Thedelivery fiber 5 is arranged for receiving at least a portion of atleast some of said broad band light pulses. In the shown embodiment thebroad band source 10 further comprises an optical component 3,preferably a filter 3 arranged between the broad band laser pulsegenerator 1 and the delivery fiber 5. The optical component 3 is forexample a polarizer, a spectral filter (preferably tunable) and/or abeam splitter.

At the output end section of the delivery fiber 5, the delivery fibercomprises a connector member 6 e.g. as described above. The connectormember 6 is advantageously configured for delivering at least a part ofsaid received portion of said broad band light pulses to an apparatus asdescribed above.

In the embodiment shown in FIG. 4 the broad band source 10 is coupled toan optical waveguide arranged for receiving light from the deliveryfiber, here called the first delivery fiber. In this embodiment theoptical waveguide arranged for receiving light from the first deliveryfiber is in form of an additional delivery assembly comprising a seconddelivery optical fiber 22 and a second delivery fiber inlet endconnector member 21 and a second delivery fiber output end terminationunit 23. The connector member 6 of the delivery fiber assembly 5, 6 inthe following referred to as the first delivery fiber assembly 5,6 isconnected to the second delivery fiber inlet end connector member 21using a mating sleeve 20.

By using a mating sleeve the connectors mechanically couple the coreregions of the first and the second delivery fibers 5, 22 so that lightcan pass from the first delivery fiber 5 to the second delivery fiber 22with low loss. Preferably the connector members are spring-loaded, sothe fiber faces are pressed together when the connector members 6, 21are mated. The resulting glass-to-glass or plastic-to-plastic contacteliminates signal losses that would be caused by an air gap between thejoined fibers.

The fiber termination unit 23 is advantageously a connector member, acollimator, a ball lens, grin lens or any other suitable terminationunit.

The second optical delivery fiber 22 can in principle be any kind ofoptical fiber, preferably having a relative broad transmission bandwidthe.g. at least about 200 nm or more, and preferably the transmissionbandwidth of the second optical delivery fiber 22 at least partiallyoverlaps the transmission bandwidth of the first delivery fiber 5.

In an embodiment the second optical delivery fiber 22 is substantiallyidentical to the first delivery fiber 5.

In an embodiment the broad band source 10 comprising the broad bandlaser pulse generator 1 and the first fiber delivery assembly 5,6 can bereplaced without replacing the second fiber delivery assembly 21, 22, 23or vice versa.

In an embodiment the broad band source 10 and said second fiber deliveryassembly 21, 22, 23 is built into an apparatus or alternatively thesecond fiber delivery assembly 21, 22, 23 is built into an apparatuswhile the broad band source 10 is arranged to feed light to theapparatus via the connection between the connector members 6, 21.Examples of such apparatus are microscopes, bio-imaging systems (such ase.g. OCT, SLO, STED, CARS and photoacoustic systems), alignment oroverlay system and manufacturing equipment (such as e.g. semiconductormanufacturing equipment). This embodiment of the invention enables thatthe broad band source 10 easily can be disconnected for service and/orcan be replaced independently of said second fiber assembly 21, 22, 23,which e.g. may be more difficult to disconnect from the remainder of theapparatus. For example the supercontinuum source and first fiberdelivery assemble constituting a broad band source of an embodiment ofthe invention can be comprised in a first module, whereas the secondfiber assembly 21, 22, 23 is part of a second module such as e.g. analignment sensor in a semiconductor wafer scribing system. In thisexample the invention enables a modular build-up of the semiconductorwafer scribing system. If the semiconductor wafer scribing system breaksdown, then the error can be located to the specific module which hasfailed, and this can be replaced independently of the other modules.This improves risk management for the semiconductor wafer scribingsystem compared to having to replace both modules at the same time.

In an embodiment the second fiber delivery assembly 21, 22, 23 is usedin bio-medical imaging or surgical applications. Examples of suchembodiments include endoscopy, colonoscopy, rhinoscopy and bronoscopy aswell as other applications where a part of the second optical fiberenters inside either a human or animal body. In such embodiments thebroad band source 10 as shown in FIG. 3 can easily be connected to theconnection member 21 as described above.

In an embodiment the second fiber delivery assembly is sterilized beforeuse.

In an embodiment the second fiber delivery assembly is disposable.

FIG. 5 is a schematic illustration of another embodiment of a broad bandsource of the invention. The broad band system 10 may be as describedabove and the second optical delivery fiber assembly 21, 22, 23 may beas described in FIG. 4.

The fiber termination unit 23 advantageously is or comprises acollimator for focusing light towards a sample 30.

Advantageously the second fiber delivery assembly 21, 22, 23 is builtinto an apparatus and the broad band source 10 is optionally arranged asa built in module in the apparatus and is arranged to feed light to theapparatus via the connection between the connector members 6, 21. Thesource 10 comprises or is optically connected to an optical detector 34,and the optical component 3 comprises an additional filter 32 arrangedto direct a portion of light 33 reflected by the sample and guided bythe fibers 5 and 22. The additional filter is advantageously a splitter.

In an embodiment the delivery fiber 5 and the second optical fiber 22are double clad fibers. Thereby a portion of the light 31 reflected bythe sample can be guided to the optical detector via the second opticaldelivery fiber assembly 21, 22, 23 and the first delivery fiber assembly5, 6. In an embodiment the delivery fiber 5 and the second optical fiber22 comprise a cladding with an NA of at least 0.1, such as at least0.15, such as at least 0.22. Also in this embodiment the fibers 5, 22will guide some of the light 31 which is being reflected from the sampleunder test.

In an embodiment the optical component 3 comprises means to separate thelight that is reflected from the sample and guided by the fibers 33 fromat least some of the light from the broad band source. Such means is forexample a beam splitter 32.

In an alternative embodiment a double-clad fiber coupler is appliedinstead of a splitter for example by providing a double-clad fibercoupler at the input end of the first delivery fiber. The double-cladfiber coupler is advantageously configured for separating core andcladding light, e.g. by having a 2×2 port structure comprising multimodedouble clad fibers on two ports of the coupler and single mode doubleclad fibers on the other two ports, such as e.g. the DC1300 LEB offeredby Thorlabs. In principle a portion or all of the reflected light couldbe separated from the major part of the light from the broad band sourcebut any other means known to the skilled person.

Advantageously the reflected and separated light is transmitted to anoptical detector 34, such as e.g. a photodiode or a spectrometer.

FIG. 6 is a schematic illustration of an embodiment of an apparatus ofthe invention in form of an interferometer e.g. for use in opticalcoherence tomography (OCT) e.g. for visualization of internal tissue.The interferometer comprises a first delivery fiber assembly 105 a, 106a comprising a fused coupler delivery fiber assembly 105 b, 106 b wherethe first delivery fiber assembly 105 a, 106 a and the fused couplerdelivery fiber assembly 106 b is fused in a component section 100. Theinterferometer comprises a broad band source comprising an optical pumpsource 101 a operable to generate pump pulses an air holemicrostructured optical fiber 101 b for generating broad band lightpulses upon feeding of pump pulses from said optical pump source 101 aand the first delivery fiber assembly 105 a, 106 a comprising a deliveryfiber 105 a comprising solid inclusions as described above and aconnector member 106 a also as described above. The first delivery fiberassembly 105 a, 106 a is connected to the microstructured optical fibervia a delivery fiber connector member 106 c which is advantageously asthe connector member described above, and an end cap connection member101 c. In the end cap connection member 101 c the air holes along lessthan a few mm of the air hole microstructured optical fiber 101 b arecollapsed and the light beam is collimated by a not shown lens. Thedelivery fiber connector member 106 c and the end cap connection member101 c are mated and held together by a mating sleeve 20 c. In analternative embodiment the air hole microstructured optical fiber 101 bis spliced to the delivery fiber 105 by splicing and/or by using a usinga GRIN lens as described above.

The interferometer comprises a second fiber assembly 21 a, 22 a, 23 awhich advantageously is as the second fiber assembly 21, 22, 23described above, and the connector members 106 a, 21 a are connected andhold together by mating sleeve 20 a.

The fused coupler delivery fiber assembly 105 b, 106 b is connected to athird fiber assembly 21 b, 22 b, 23 b which advantageously is as thesecond fiber assembly 21, 22, 23 described above, and the connectormembers 106 b, 21 b are connected and held together by mating sleeve 20b. The interferometer further comprises a mirror 40 or another referenceunit arranged to reflect light emitted via the fiber termination unit 23b. In an alternative embodiment the reference unit is not included inthe apparatus but can be selected by the user.

The fused coupler delivery fiber assembly 105 b is further connected toa detector 124, such as a spectrometer. The broad band source maycomprise one or more tunable or non-tunable filters and or a pulsepicker and one or more amplifiers such as it is well known in the art.

In general a coupler has a bar port, where the light goes straightthrough from one top arm to the other top arm (or from the bottom arm tothe other bottom arm), and a cross port where the light goes from thetop arm to bottom arm, or vice versa. Often couplers are close to have avery low loss such that all the light is send to the bar port or thecross port. In this embodiment the two top arms are provided by thedelivery fiber 105 a on either side of the component section 100 and thebottom arms are provided by the delivery fiber 105 b on either side ofthe component section. The bar ports and the cross port are provided bythe component section 100. The bar port has a transmission coefficientof x and the cross arm a transmission coefficient of (1-x). Thetransmission coefficient is the same irrespective of which direction thecoupler is traversed.

In use broad band light pulses are transmitted to the delivery fiber 105a and as marked on the illustration in the end of the delivery fiber 105a nearest to the microstructured optical fiber the light pulses power isset to be 100%=“1”. At the fused component section 100, some of thelight (x) is transmitted further via the delivery fiber 105 a and someof the light (1-x) is transmitted further via the fused coupler deliveryfiber 105 b.

The light portion (X) transmitted from the fused component section 100and via the delivery fiber 105 a is transmitted to the second fiberassembly 21 a, 22 a, 23 a and via the fiber termination unit 23 a thelight pulses are emitted towards a sample 30 and reflected light 31 a istransmitted in the opposite direction via the second fiber 22 a and thedelivery fiber 105 a until the remitted light reaches the fusedcomponent section 100. From there a portion of remitted light istransmitted further via the fused coupler delivery fiber 105 b to thedetector 104.

The light portion (1-X) transmitted from the fused component section 100and via the fused coupler delivery fiber 105 b is transmitted to thethird fiber assembly 21 b, 22 b, 23 b and via the fiber termination unit23 b the light pulses are emitted towards a mirror 40 and reflectedlight 31 b is transmitted in the opposite direction via the third fiber22 b and the fused coupler delivery fiber 105 b until the remitted lightreaches the fused component section 100. From there a portion ofremitted light is transmitted further via the fused coupler deliveryfiber 105 b to the detector 104. Advantageously the mirror reflectssubstantially all of the light that is incident on it.

As explained the interferometer thereby has two interferometer arms, onethat is guiding light to a sample and re-transmit reflected light andone that is guiding light to a reference unit (e.g. a mirror) andre-transmits reflected light. In an embodiment one interferometer arm isconfigured for being focused onto a tissue sample and for scanning thesample in an X-Y longitudinal raster pattern. The other interferometerarm is bounced off the reference mirror.

Reflected light from the tissue sample is combined with reflected lightfrom the reference.

As mentioned above in the embodiment shown in FIG. 6 the light goes fromthe broad band laser goes through the cross port of the coupler to reachthe mirror 40. It is reflected from the mirror and goes back through thebar port of the coupler to reach the detector 124. Assuming that thereflection is loss less the transmission coefficient for the entire pathis the product of the two transmission coefficients, i.e. x(1-x).

FIG. 7 is a schematic illustration of an embodiment of a delivery fiberof the invention comprising a combiner.

Advantageously all of the delivery fibers 105 a, 105 b, 22 a, 22 b areall solid fibers comprising microstructures in form of inclusions asdescribed above and preferably all of the delivery fibers 105 a, 105 b,22 a, 22 b have a transmission bandwidth of 200 nm or more, preferablythe transmission bandwidths are overlapping or identical.

FIG. 7 is a schematic illustration of an embodiment of an apparatus ofthe invention in form of another type of interferometer e.g. for use inoptical metrology e.g. for thin film, wafer, optical critical dimension(OCD), overlay and wafer stress for transistor and interconnectmetrology applications.

Parts of the interferometer of FIG. 7 are similar to corresponding partsof the interferometer of FIG. 6. The interferometer comprises a firstdelivery fiber assembly 105 a, 106 a comprising a fused splitterdelivery fiber assembly 105 b, 106 b where the first delivery fiberassembly 105 a, 106 a and the fused splitter delivery fiber assembly 106b is fused in a component section 100. The interferometer comprises abroad band source comprising an optical pump source 101 a operable togenerate pump pulse, a microstructured optical fiber 101 b forgenerating broad band light pulses upon feeding of pump pulses from saidoptical pump source 101 a and the first delivery fiber assembly 105 a,106 a comprising a delivery fiber 105 a comprising solid inclusions asdescribed above and a connector member 106 a also as described above.The first delivery fiber assembly 105 a, 106 a is connected to themicrostructured optical fiber via a filter 32 arranged to directing aportion of reflected light 33 reflected by the sample 30 e and thereference unit 40 towards an optical detector 34. The remaining partsare as in the example of FIG. 7.

FIG. 8 illustrates a spectral engine source suitably for supplying lightto an apparatus. The spectral engine source comprises three lasers 121,222, 123 emitting laser beams 121 a, 122 a, 123 a respectively. Thelaser beams 121 a, 122 a, 123 a differs from each other with at leastone wavelength as described above.

The lasers 121, 222, 123 may be of same or of different types, such asone or more gas lasers, one or more chemical lasers, one or moremetal-vapor lasers and/or one or more semiconductor lasers.

The spectral engine source further comprises a multiplexer M hereillustrated by a number of mirrors arranged to combine the beams 121 a,122 a, 123 a into one single multiplexed beam M. It should be understoodthat the multiplexer may be any kind of multiplexer or combiner capableof combining least a portion of the laser beams of each of the lasersand for collimating the received light to a multiplexed beam. Themultiplexer M collimates the 121 a, 122 a, 123 a sufficiently close tobe received by the delivery fiber of the spectral engine source of theinvention. For simplification the delivery fiber is not shown of thespectral engine source, however the delivery fiber is arranged tocollect the multiplexed beam M and to delivering at least a part of thereceived multiplexed beam M to the apparatus,

FIG. 9 illustrates another spectral engine source suitably for supplyinglight to an apparatus. The spectral engine source comprises 5 lasers131, 232, 133, 134, 135 emitting laser beams 131 a, 132 a, 133 a, 134 a,135 a respectively. The laser beams 131 a, 132 a, 133 a, 134 a, 135 adiffers from each other with at least one wavelength. For example laserbeam 131 a may comprise wavelength (s) in the range of 400-500 nm, laserbeam 132 a may comprise wavelength (s) in the range of 500-600 nm, laserbeam 133 a may comprise wavelength (s) in the range of 600-700 nm, laserbeam 135 a may comprise wavelength (s) in the range of 800-900 nm.

The spectral engine source further comprises a multiplexer M hereillustrated by a number of mirrors arranged to combine the beams 131 a,132 a, 133 a, 134 a, 135 a into one single multiplexed beam M. Thespectral engine source also comprises a not shown delivery fiber asdescribed above for receiving the multiplexed beam and to delivering atleast a part of the received multiplexed beam M to an apparatus,

FIG. 10 illustrates an apparatus of an embodiment of the inventioncomprising a light source LS selected from a broad band source, a broadband source system or a spectral engine source with a delivery fiber DLas described above. The apparatus is a lithographic apparatus andcomprises an illumination system (illuminator) IL arranged to receivethe light from the delivery fiber of the light source LS. Theilluminator IL may comprise an adjuster AD for adjusting the angularintensity distribution of the radiation beam received from the lightsource LS. In addition, the illuminator IL may comprise various othercomponents, such as an integrator IN and a condenser CO. The illuminatormay be used to condition the radiation bean, to have a desireduniformity and intensity distribution in its cross-section.

The radiation beam B is incident on a patterning device (e.g., mask MA),which is held on the support structure (e.g., mask table MT), and ispatterned by the patterning device. Having traversed the mask MA, theradiation beam B passes through the projection system PS, which focusesthe beam onto a target portion C of the substrate W. With the aid of thesecond positioner PW and position sensor IF (e.g. an interferometricdevice, linear encoder or capacitive sensor), the substrate table WT canbe moved accurately, e.g. so as to position different target portions Cin the path of the radiation beam B. Similarly, the first positioner PMand another position sensor (which is not explicitly depicted in FIG. 1)can be used to accurately position the mask MA with respect to the pathof the radiation beam B, e.g. after mechanical retrieval from a masklibrary, or during a scan. In general, movement of the mask table MT maybe realized with the aid of a long-stroke module (coarse positioning)and a short-stroke module (fine positioning), which form part of thefirst positioner PM. Similarly, movement of the substrate table WT maybe realized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the mask table MT may be connected to ashort-stroke actuator only, or may be fixed. Mask MA and substrate W maybe aligned using mask alignment marks M1, M2 and substrate alignmentmarks P1, P2. Although the substrate alignment marks as illustratedoccupy dedicated target portions, they may be located in spaces betweentarget portions (these are known as scribe-lane alignment marks).Similarly, in situations in which more than one die is provided on themask MA, the mask alignment marks may be located between the dies. Theapparatus may for example operate as described in US2007013921

FIG. 11 illustrates an apparatus of an embodiment of the inventioncomprising scanning microscope that is embodied as a confocal scanningmicroscope and a light source LS selected from a broad band source, abroad band source system or a spectral engine source with a deliveryfiber DL as described above. The light source LS delivers anilluminating light beam 211 via the delivery fiber DL

The scanning microscope comprises an acoustooptical component 213 thatis embodied as AOTF 215. From acoustooptical component 213, light 212,selected out of illuminating light beam 211, arrives at a beamdeflection device 17 that contains a gimbal-mounted scanning mirror 19and that guides illuminating light beam 211 through scanning opticalsystem 221, tube optical system 223, and objective 225 over or throughspecimen 227. Detected light beam 229 coming from the specimen travelsin the opposite direction through scanning optical system 221, tubeoptical system 223, and objective 225, and arrives via scanning mirror219 at acoustooptical component 213 which conveys detected light beam229 to detector 231, which is embodied as a multi-band detector.Illuminating light beam 211 is depicted as a solid line in the drawing,and detected light beam 229 as a dashed line. Illumination pinhole 233and detection pinhole 235 that are usually provided in a confocalscanning microscope are schematically drawn in for the sake ofcompleteness. Omitted in the interest of better clarity, however, arecertain optical elements for guiding and shaping the light beam.Acoustooptical component 213, which serves to select the wavelengthspectrum that is chosen, is configured as AOTF 215, through which anacoustic wave passes. The acoustic wave is generated by an electricallyactivated piezo acoustic generator 237. Activation is accomplished by ahigh-frequency source 239 that generates an electromagnetichigh-frequency wave that exhibits an adjustable HF spectrum.

The HF spectrum is chosen in such a way that only those portions ofilluminating light beam 211 having the desired wavelength arrive at beamdeflection device 217. The other portions of illuminating light beam 211not influenced by the acoustic excitation are directed into a beam trap241. The power level of the illuminating light beam 211 can be selectedby varying the amplitude of the acoustic wave. The crystal sectioningand orientation of acoustooptical component 213 are selected in such away that with a single coupling-in direction, different wavelengths aredeflected in the same direction. A computer 243 is used to choose asecond or third wavelength spectrum. Monitor 247 of computer 243 servesas the display for the spectral composition. Selection of the wavelengthspectrum together with its spectral composition is accomplished on thebasis of a graph G within a coordinate system having two coordinate axesX, Y. The wavelength of the light is plotted on coordinate axis X, andits power level on coordinate axis Y. Computer 243 controlshigh-frequency source 239 in accordance with the user's stipulation. Theuser makes adjustments using computer mouse 257. Depicted on monitor 247is a slider 259 that serves for adjustment of the overall light powerlevel of illuminating light beam 11 or detected light beam 229.

Although embodiments have been described and shown in detail, theinvention is not restricted to them, but may also be embodied in otherways within the scope of the subject matter defined in the followingclaims. In particular, it is to be understood that other embodiments maybe utilized and structural and functional modifications may be madewithout departing from the scope of the present invention.

The invention claimed is:
 1. A delivery fiber assembly configured fordelivering broad band light and comprising a microstructured deliveryfiber and a connector member, said delivery fiber having a longitudinalaxis, an input end for launching light and a delivery end for deliveringlight, said delivery fiber comprises a core region and a cladding regionsurrounding the core region wherein the cladding region comprises acladding background material having a refractive index Nbg and aplurality of inclusions of solid material having a refractive index upto Ninc and extending in the direction of longitudinal axis of thedelivery fiber, wherein Ninc<Nbg and the plurality of inclusions in thecladding region is arranged in a cross-sectional pattern comprising atleast two rings of inclusions surrounding the core region, said coreregion having a diameter up to about 15 μm, said connector member beingmounted to said delivery fiber at a delivery end section of the deliveryfiber, said delivery fiber defining a bandwidth of about 200 nm or morein which a transmission loss is less than 0.5 dB/m at all wavelengthswithin the bandwidth.
 2. The delivery fiber assembly of claim 1, whereinsaid connector member is mounted to said delivery fiber at said deliveryend section of the delivery fiber at a distance to said delivery end,such that the delivery end is passing through the connector member forconnecting said delivery end in physical contact with a receiverwaveguide.
 3. The delivery fiber assembly of claim 1, wherein themicrostructured delivery fiber for at least one wavelength within saidbandwidth and at least along said delivery end section has a constantmode field diameter.
 4. The delivery fiber assembly of claim 1, whereinthe microstructured delivery fiber at least along said delivery endsection has a mode field diameter range for said bandwidth of about 200nm or more, wherein the mode field diameter range is about 30% or lessof the lowest mode field diameter of the range.
 5. The delivery fiberassembly of claim 1, wherein said microstructured delivery fiber issingle mode for at least one wavelength within said bandwidth.
 6. Thedelivery fiber assembly of claim 1, wherein the microstructured deliveryfiber has a core region diameter to pitch d/λ which is about 0.7 orless.
 7. The delivery fiber assembly of claim 1, wherein the pluralityof inclusions in the cladding region is arranged in a cross-sectionalpattern comprising at least four rings of inclusions surrounding thecore region.
 8. The delivery fiber assembly of claim 1, wherein themicrostructured delivery fiber is a silica fiber and the solidinclusions comprise down doped silica.
 9. The delivery fiber assembly ofclaim 1, wherein an inclusion of said inclusions has a higher refractiveindex than another inclusion of said inclusions.
 10. The delivery fiberassembly of claim 1, wherein an inclusion of said inclusions has adifferent diameter than another inclusion of said inclusions.
 11. Thedelivery fiber assembly of claim 1, wherein the assembly furthercomprises an input end connector member, the input end connector memberbeing mounted to said microstructured delivery fiber at an input endsection of the microstructured delivery fiber comprising said input end,said input end connector member being configured for connecting saidinput end in physical contact with a light launching unit.